1. Field of the Invention
The present invention relates to a variable inductor incorporated e.g. in radio communications equipment.
2. Description of the Related Art
In the technical field of radio communications equipment such as mobile phones, there is an increasing requirement for smaller high-frequency circuits or RF circuits due to increase in the number of parts incorporated in the equipment for advanced features. In response to such a requirement, a variety of parts needed for building the circuitry are a focus of miniaturization using technologies called MEMS (micro-electromechanical systems). Inductors are a category of such parts. Inductors are an electronic part to be incorporated in electric circuits or electronic circuits, for use of an inductance provided by them, and sometimes there is a need for the inductance to be variable.
FIG. 30 and FIG. 31 show a primary configuration of an inductor X4 which is a conventional variable inductor whose inductance is variable. FIG. 30 is a plan view of the inductor X4 whereas FIG. 31 is a sectional view taken in lines XXXI—XXXI in FIG. 31.
The inductor X4 includes a substrate 91, a conductor 92 and a ferrite core 93. The conductor 92, which is formed on the substrate 91 using such technologies as thin-film formation and patterning technology, has an electroconductive coil 92a and a pair of terminals 92b. The ferrite core 93 has a high magnetic permeability and faces the coil 92a. Further, the ferrite core 93 is movable toward and away from the substrate 91 or the coil 92a within a predetermined range of movement. Such a variable inductor is disclosed in e.g. the following Patent Document 1.
Patent Document 1: JP-A-H08-204139
In the inductor X4, the ferrite core 93 is brought closer to the coil 92a in order to increase the inductance (self inductance) between the pair of terminals 92b in the inductor X4. When the ferrite core 93 is moved away from the coil 92a, the inductance is decreased. The coil's self inductance is known to be proportional to magnetic permeability in which the coil is placed. The closer is the distance between the ferrite core 93 and the coil 92a, higher is the net magnetic permeability in the environment around the coil 92a (and therefore higher is the net density of the magnetic flux generated around the coil 92a in association with an electric current flowing through the coil 92a), and so the inductance is higher.
However, in the inductor X4 whose inductance is varied by an advancing/retracting movement of a magnetically highly permeable member (the ferrite core 93) with respect to the coil 92a, the inductance can only be varied within a relatively small range of 10% approx, as mentioned in the Patent Document 1. Therefore, the inductor X4 is sometimes unable to vary its inductance as much as desired.